1. Field of the Invention
The present invention relates to a token-ring-type local area network (LAN) wherein data transfer is carried out among plural communication nodes which are connected together by a communication link.
2. Prior Art
As the conventional LAN communication method in which information can be treated in real-time manner, the following methods are provided.
(1) Time-Stamp Method
In each communication node, the data is to be transferred with the generation time (i.e., time-stamp) thereof.
(2) Line Switching Method
By use of the line-switching-type LAN which is operated according to the frequency-band-sharing or time-sharing method, one line is assigned between two communication nodes. Herein, at least one line is used to control the line assignment by use of the packet-switching-type protocol.
(3) Token-Passing Method
This is the method in which each communication node obtains the token circulating around the communication path to thereby obtain the transmission right. This method has the following features.
(a) The transmitting node is controlled by the token, by which it is possible to avoid the so-called conflicting transmission states (or collision state) by plural communication nodes.
(b) The period in which the token is retained is limited, by which the tokens are sequentially transferred from one communication node to the next communication node. Thus, each communication node can be operated with an even chance.
(c) By providing a priority to the token, the high-priority data can be transferred at first.
(4) Hybrid Method
This method is characterized by using two different protocols. This method can be further classified into two methods. In a first method, there are provided physically double rings, each having a different real-time response. In a second method, there is provided a single ring, however, there are provided two protocols in which one is operated on a real-time basis and the other is operated on a non-real-time basis. In this case, one of two protocols are adequately used.
Meanwhile, the above-mentioned conventional LAN is constructed for the field of file exchange or key-stroke communication of terminals, which do not require the real-time response.
However, especially in the field of the electronic musical instruments and the like, the real-time response is required in order to improve the performance expression. Further, the automatic performance requires the ability to receive large-scale sequence information for the performance from the file server while the sequencer transmits the performance information to the musical instrument. In other words, it requires the real-time response and large-scale communication commonly.
The above-mentioned demand is not limited to the field of the music performance. In short, such demand can be considered as the fundamental demand which must be satisfied in the future LAN.
However, there are some drawbacks by which the conventional LAN cannot respond to such demand sufficiently as follows.
For example, the foregoing time-stamp method (see (1)) can satisfy the demand in case of the recording of the performance.
On the other hand, the foregoing line switching method (see (2)) can directly satisfy the demand, however, there is another drawback in which it is difficult to determine the number of lines to be provided in advance. Because, this method is designed on the basis of the ideal concept in which it is not so affected by the increase of the nodes and the ability thereof is reduced continuously in response to the data quantity.
Further, the packet-switching-type LAN can avoid the transmission collision of plural nodes and also perform the even-chance processing and priority processing, which is superior in the real-time response. Particularly, the token-ring-type LAN is not so affected by the increase of the nodes and the ability thereof is reduced continuously in response to the data quantity.
In this case, the real-time response depends on the period in which the token is retained in each communication node. However, reduction of this period results in a drawback in that the communication efficiency must be reduced when transferring large-scale data.
Moreover, the hybrid method (see (4)), in which the physically double token rings are used, is disadvantageous because of the complicated facility and high cost required. In addition, the method in which two protocols are used is disadvantageous because of the complicated control and difficult maintenance to be required.
Meanwhile, in order to synchronize operations of plural communication nodes with the common clock, one clock source simultaneously supplies the fundamental clock to plural communication nodes in a real-time manner. However, the conventional token-ring-type LAN requires the real-time transfer of the clock. In this case, if an error has occurred, it is impossible to re-transfer the clock. For this reason, the conventional LAN does not provide the efficient error correction, e.g., means for compensating the clock error. Thus, there is a demand to provide the clock sharing method in the LAN.